A mule is a hybrid, generally sterile, produced by the breeding of a male donkey with a female horse. The result varies widely, depending of course on the mare to which the donkey is bred, horses varying far more widely than donkeys in size and conformation.
Their combination of qualities, however, is very useful: inheriting its size, its muscle and its speed from its dam, a mule can cover a lot of ground, and it requires less food to do so. It is tougher in many ways — its skin is less sensitive, its smaller hooves harder — and it has great endurance. This toughness and surefootedness are inherited from the donkey sire, along with its disposition.
That donkey disposition gives the mule its proverbial stubbornness: it is cautious, like a donkey, reluctant to commit itself to anything new, and unafraid to express its reluctance.
Mules have their admirers, chief among them those who need intelligent beasts of burden – which would include the army. Charles Darwin thought them a triumph of hybrid vigour:
The mule always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, powers of muscular endurance, and length of life, than either of its parents, seems to indicate that art has here outdone nature. (1)
(1) What Mr. Darwin saw in his voyage round the world in the ship ‘Beagle’. New York, 1879. 34.
The image of mules expressing their resistance to anything new is a cartoon by James Frise, from Kay, Hugh, George Magee and F.A.MacLennan. Battery Action! The Story of the 43rd Battery CFA. Toronto: . 98.